Conjugated diene monomers are often anionically polymerized by using alkyllithium compounds as initiators. Selection of certain alkyllithium compounds can provide a polymer product having a functionality at the head of the polymer chain. The ability to head-functionalize anionically-polymerized polymers has provided many advantages to tire technology. For example, lithiated cyclic amines, such as lithio hexamethyleneimine, has been employed to initiate the polymerization of conjugated dienes, as well as the copolymerization of conjugated dienes and vinyl aromatic monomers, to produce polymers having a cyclic-amine head functionality. These polymers have proven to provide technologically useful tire treads that are characterized by improved traction, low rolling resistance, and improved wear.
The synthesis of these polymers is advantageously conducted in environmentally friendly solvents such as technical hexanes. The high temperatures at which some polymerizations occur, however, has led to the problem of reduced head functionality. To alleviate this problem, it was discovered that the use of cyclic aminoalkyllithium compounds, such as hexamethyleneimine propyllithium, could withstand high polymerization temperatures and thereby lead to polymers having greater functionality.
The preparation of these cyclic aminoalkyllithium compounds, however, has proven to be difficult and inefficient. In one technique, the initiators are prepared by reacting a cyclic aminoalkyllithium halide with elemental lithium or an organolithium compound. Where the halide is reacted with elemental lithium, the product must be separated from byproducts such as lithium metal and lithium chloride mud. Separation of these products can prove difficult, in part due to the limited solubility of the aminoalkyllithiums. Additionally, the product made by this route is often contaminated with undesirable side products such as the products of Wurtz coupling. Moreover, the precursor aminoalkylhalide compounds are capable of self-quaternization, thus consuming the reactive halide. As a result, it is necessary to store these compounds at low temperatures or as their hydrohalide salts and liberate the aminoalkylhalide by treatment with base a short time before lithiation is carried out. When the halide is reacted with an organolithium compound, the reaction inefficiently requires two or more equivalents of lithium from the organolithium to prevent undesirable side reactions that occur between the lithiated amine and the resultant chlorinated organic byproducts.
Because cyclic aminoalkyllithium compounds remain useful as initiators for preparing functionalized polymers, there is a need to overcome the problems associated with the synthesis of these initiators.